Egyptian security forces arrested rights lawyer Malek Adly and political activist Zizo Abdo in the district of Maadi in Cairo on Thursday night.
A prosecutor questioned Adly and ordered he be detained for 15 days pending investigations into charges that include attempting to overthrow the regime and inciting the 25 April protests.
He is also being investigated on charges of spreading false news and harming national unity and societal peace. Adly denied all charges during the questioning.
The 25 April protests condemned the government's decision to recognise two Red Sea islands, administered by Egypt, as belonging to Saudi Arabia. The decision has yet to be ratified by Egypt's parliament.
Abdo was transferred from Maadi police station to Giza police station, according to Freedom for the Brave rights group, and has yet to be questioned by a prosecutor.
Hundreds were arrested in the lead-up to the protests and during the demonstrations themselves.
Protesting without a police permit is a crime in Egypt, and violators are often sentenced to jail terms.